Multilayer integrated optical device and a method of fabrication thereof

ABSTRACT

A method of fabricating an integrated optical device and such a device, comprising a structure including at least one waveguiding element are presented. A basic structure is formed containing a substrate material carrying a buffer material layer coated with a core material layer of a higher refraction index as compared to that of the buffer layer. The at least one waveguiding element is defined in a guiding layer on top of the basic structure. The guiding layer is made of a material with a refractive index higher than the refractive index of the buffer layer and the core layer, and is chosen so as to minimize a height of the at least one waveguiding element and to provide effective guiding of light in the core layer. A cladding layer is formed on top of the so-obtained structure, wherein a height difference between the cladding layer region above the waveguiding element and the cladding layer region outside the waveguiding element is substantially small resulting in a desired flatness of the top cladding layer to allow direct formation of a further waveguide structure thereon and prevent significant perturbations in light propagation within the further waveguide structure.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisionalapplication No. 60/232,061, filed Sep. 12, 2000.

FIELD OF THE INVENTION

This invention is in the field of Planar Lightwave Circuits (PLC), andrelates to a multilayer integrated optical device and a method offabrication thereof.

BACKGROUND OF THE INVENTION

Optical communications is the enabling technology for the informationage, and the essential backbone for long haul communications. As thistechnology progresses, there is a tremendous interest in providingoptical routes in the short haul, metropolitan and access networks, aswell as in local area networks and cable TV networks. In all thesenetworks, the best of breed solution for bandwidth expansion has beenthe adoption of wavelength division multiplexing (WMD), which entailsthe aggregation of many different information carrying light streams onthe same optical fiber. Devices capable of accessing individualinformation streams are fundamentally required in current and futurenetworks. These devices can also add information streams to the opticalfiber, as well as impress information on an optical stream by opticalmodulation.

PLC technology is central in the creation of modern optical elements forcommunications systems. According to this technology, optical waveguidesand additional functional structures are fabricated in a planar opticaltransparent medium in order to direct the passage of light and toimplement coupling, filtering, switching, and additional processingfunctions as required for optical communications.

Existing examples of Planar Lightwave Circuits include optical switchesand modulators based on Mach Zender Interferometer (MZI), in whichinterference is produced between phase coherent light waves that havetraveled over different path lengths; arrayed waveguide routers (AWG)used for combining and spreading multiple optical channels, namelymultiplexers and demultiplexers. However, to achieve a good modulationperformance with the MZI, the latter is typically designed with longinterference arms. As a result, this device is not size-efficient in itsimplementation, and limits the scaling ability of complex opticalcircuits. Another feature of MZI-type devices, in the predominantimplementations, is their frequency insensitivity over a desiredfrequency bandwidth. As a result, MZI-type devices cannot be useddirectly for wavelength routing.

An important driving force pushing PLC technology is the need forenhanced functionality in the optical domain. This need is hampered bythe limitation of state of the art waveguide technology, which istwo-dimensional (i.e., single-functional-layer architecture). Unlike thevery large scale integrated electronic circuitry, where dimensions ofthe basic elements were reduced to sub-micrometer size, the optical PLCcircuitry is inherently much larger, thus the exploitation ofmulti-layer architectures is more crucial than in electronics.

In the implementation of PLC, there is a contradiction between therequirements of coupling to optical fibers and decreasing circuit size.Coupling to fibers is best obtained by using waveguides with modalfields similar to the fiber modes with a small refractive indexdifference with respect to the surrounding medium. The functionality ofthe optical circuits depends on the amount of optical element in thecircuit. By decreasing the circuit size, more optical circuits can beintegrated and the attainable functionality increases. Smallerdimensions imply tighter control of the optical mode and smaller opticalmodes, hence, a high index contrast between the waveguide core andsurrounding medium. It is of fundamental importance to provide a meansof combining both elevens in one functional optical circuit.

The importance of utilizing the vertical dimensions in creating complexoptical circuits has been recognized and addressed in the past. This isassociated with the fact that vertical fabrication tolerances are betterthan horizontal tolerances, and therefor such a vertical integratedoptical device (filter, switch, modulator) is simpler or cheaper tomanufacture. Optical devices utilizing this approach are disclosed, forexample, in the article “Vertically Coupled Glass Microring ResonatorChannel Dropping Filter”, B. E. Little et al., IEEE Photonics technologyLetters, Vol. 11, No. Feb. 2, 1999. This approach is critical for thefabrication of optical circuits based on structures with very differentindices of refraction such that the effective coupling region betweenthe structures is very small, e.g., coupling between waveguides and ringmicro-resonators. In this case, the vertical dimension, which is easierto control in conventional processes, can mediate the structure foraccurate coupling as described in the aforementioned reference.

Recently developed integrated electro-optical devices utilize resonantrings to achieve frequency selective switching. Such a device isdisclosed, for example, in WO 99/17151. The device comprises a ringresonator interconnected by linear waveguides to couple light from afirst linear waveguide to the second one, when the frequency of thelight passing through the first waveguide fulfils that of the resonancecondition of the ring. By applying an electric field to the ring, itsrefractive index, and consequently, its resonance condition, can bedesirably adjusted, thereby preventing the passage of the previouslycoupled light, the device therefore acting as a switch. Alternatively,the loss of the ring waveguide can be changed. Adding loss to the ringdiminishes its operation as a resonant cavity, and light cannot becoupled from the waveguide to waveguide.

To create two or more layers of interconnected waveguides with the priorart techniques, a planarization step has to be performed. FIGS. 1A-1Dillustrate main sequential steps of the prior art technique employed formanufacturing a waveguide structure shown in FIG. 1E being generallydesignated 10. Initially, a buffer layer 12 of SiO₂ is deposited on asilicon wafer 14 (FIG. 1A). Then, a layer 16 of doped SiO₂ with a higherrefractive index (SiO₂+Ge), as compared to that of the buffer layer 12,is deposited onto the buffer layer (FIG. 1B). This layer 16 serves forthe formation of a core 16A of the optical waveguide, and its thicknessis typically in the range of 4-12 μm. To form the waveguide core 16A(FIG. 1C), the waveguide, as well as other optical structures, aremasked using photolithography followed by etching. A third layer ofSiO₂, or upper cladding layer 18, is then deposited so as to bury theetched structure (FIG. 1D).

This layer 18 retains to some extent the topography of the underlyingstructure, and thus requires planarization to allow for an additionaloverlaying waveguide structure to be deposited. Planarization can beimplemented by Chemical Mechanical Polishing, reflow techniques,deposition of a very thick layer, selective etching or depositiontechniques. As shown in FIG. 1F, after achieving a planar top layer, asecond waveguide structure 20 can be fabricated on top of the structure10 in the above-described manner.

Planarization is a difficult process step, which utilizes expensiveequipment and is difficult to be accurately applied for larger areawafers. Therefore, it would be desirable to eliminate this step in thefabrication of multi-layered optical waveguide structures.

SUMMARY OF THE INVENTION

There is accordingly a need in the art to facilitate the manufacturingof a three-dimensional (multi-layered) integrated optical device, byproviding a novel method of fabricating such a device, and a novelintegrated optical device based on an optical structure embodyingdifferent material systems. Such a device may be an optical frequencydependent switch, a modulator, an Optical Add Drop Multiplexer (OADM), aspectral analyzer, a sensor, etc.

The main idea of the present invention consists of utilizing a waveguidedefinition on several layers, enabling to combine low coupling losswaveguides with high confinement waveguides. The present invention opensnew horizons for the functionality of optical device using PlanarWaveguide Technology. The present invention provides a fabricationmethod for the manufacture of the three-dimensional fabrics ofwaveguides with three-dimensional interconnections. Furthermore, sinceit is recognized that three-dimensional interconnections are crucial forcreating resonator-based, high-density optical circuits, the inventionprovides a fabrication method for such devices.

The invention provides for the fabrication of three-dimensional opticalwaveguiding structures by simple processing steps. The main innovationhere relates to the elimination or at least alleviation of theplanarization step, which is difficult to implement. As indicated above,planarization is required to overcome the perturbations in a given layercaused by the previously deposited layers. In the present invention, theadopt of novel waveguide structures minimizes the perturbation, andfacilitates multi-level integration of light guiding structures.

There is thus provided according to one aspect of the present invention,a method of fabricating an integrated optical device comprising astructure including at least one waveguiding element, the methodcomprising the steps of:

(i) forming a basic structure containing a substrate material carrying abuffer material layer coated with a core material layer of a higherrefraction index as compared to that of the buffer layer;

(ii) defining said at least one waveguiding element in a guiding layeron top of said basic structure, wherein said guiding layer is made of amaterial with a refractive index higher than the refractive index ofsaid buffer layer and the core layer, and is chosen so as to minimize aheight of said at least one waveguiding element and to provide effectiveguiding of light in the core layer;

(iii) forming a cladding layer on top of a structure obtained in step(ii), wherein a height difference between a height of the cladding layerregion above said at least one waveguiding element and a height of thecladding layer region outside the waveguiding element is substantiallysmall, thereby providing a sufficient flatness of the top cladding layerto allow formation of a further waveguide structure thereon and preventsignificant perturbation in light propagation within said furtherwaveguide structure.

At least one waveguide element may be defined by a ridge of the highindex material (as compared to the buffer layer) on top of the basicstructure. Alteratively, this waveguide element may be a resonator ring(resonator cavity loop), in which case the further waveguide structurecontains a further waveguiding element formed on top of the claddinglayer by repeating steps (i) and (ii).

It should be understood that the term “sufficient flatness of thecladding layer” used herein signifies a flatness defined by the heightdifference of the different regions of the cladding layer (i.e., abovethe waveguiding element and outside the waveguiding element) muchsmaller than the optical mode size of a further waveguiding element.

For example, the height difference in the order of few hundreds ofnanometers can be obtained, the optical mode size of the furtherwaveguide formed on the cladding layer being about several micrometers.Typically, the height difference of the cladding layer does not exceedthe height of the at least one waveguiding element (i.e., the thicknessof the guiding layer) covered by said cladding layer.

According to another aspect of the present invention, there is provideda method of fabricating a three-dimensional integrated optical devicecomprising at least two vertically aligned waveguide structures, eachincluding at least one waveguiding element, the method comprising thesteps of:

(a) forming a basic structure containing a substrate carrying a bufferlayer coated with a core material layer of a higher refractive index ascompared to that of the buffer layer;

(b) defining said at least one waveguiding element of the lowerwaveguide structure in a guiding layer on top of said basic structure,wherein said guiding layer is made of a material with a refractive indexhigher than the refractive index of said buffer layer and the core layeris chosen so as to minimize a height of said at least one waveguideelement and to provide effective guiding of light in the core layer; and

(c) forming an upper cladding layer on top of a structure obtained instep (b), wherein a height difference between a height of the claddinglayer region above said at least one waveguiding element and a height ofthe cladding layer region outside the waveguiding element issubstantially small, thereby providing an sufficient flatness of saidupper cladding layer to allow direct formation of the upper waveguidestructure thereon;

(d) forming said upper waveguide structure on top of said upper claddinglayer by depositing a further buffer layer and repeating step (b) and(c) with respect to a further guiding layer, significant perturbation inlight propagation within said upper waveguide structure being therebyprevented.

According to yet another aspect of the present invention, there isprovided an integrated optical device comprising at least one structurehaving at least one waveguiding element, the device comprising:

a basic structure containing a substrate material carrying a buffermaterial layer coated with a core material layer of a higher refractionindex as compared to that of the buffer layer;

said at least one waveguiding element formed in a guiding layer on topof said basic structure, wherein said guiding layer is made of amaterial with a refractive index higher than the refractive index ofsaid buffer layer and the core layer, and is chosen so as to minimize aheight of said at least one waveguiding element and to provide effectiveguiding of light in the core layer;

a cladding layer on top of a structure with said at least onewaveguiding element, wherein a height difference between a height of thecladding layer region above said at least one waveguiding element issubstantially small, the top cladding layer thereby having a desiredflatness.

The device may comprise additional waveguides and additionalloop-resonators, forming together several frequency selective switches,thereby providing complex optical signal switching and routing.

Since optical waveguides can be implemented in complex manners, theuniversal quantity characterizing the behavior of the confined light isthe effective refractive index of the waveguide. In conventionaldevices, the difference between the effective refractive index of thewaveguide and the index of the surrounding medium is typically smallerthan 1%. When using ring micro resonator structures, the effectiverefractive index of the ring waveguide has to be large, i.e., typicallygreater than 20%, to accommodate tight mode confinement and smalllosses. In these structures, however, the effective index of the ringwaveguide and the linear waveguide are similar to within few percents(e.g., about 30%). The present invention provides for using several (atleast two) ring resonators (ring waveguides) in an integrated opticaldevice, the refractive index of the ring waveguide being therebysubstantially greater (e.g., 20% greater) than the refractive index ofthe linear waveguide that receives an input signal.

In an optical complex filter/resonator according to the invention,waveguide sections are specifically connected to ring resonators in aconfiguration which enables realization of optical switching, wavelengthrouting, optical filtering, etc. The device may also combine a pluralityof such filters in a wavelength router module.

Modern optical communications are typically based on transmittingfrequency multiplexed optical signals through an optical fiber. The OADMis capable of adding or dropping optical channels from an optical fiber,and is an essential element in modern optical communications. In thepresent invention, the OADM is based on a combination of tunablefilters, which provide the add or drop multiplexing functions. SinceOADMs have to meet stringent criteria in the filtering, each ringresonators is an optical filter, and, by combining them in parallel,high order filters are obtained.

In general, the resonator-cavity loops (ring-resonators) can be replacedby any other implementation of a frequency-selective element that couplebetween the two waveguide sections. For example, optical gratings can beused.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIGS. 1A to 1F illustrate the prior art technique of manufacturing awaveguide structure;

FIGS. 2A and 2B compare the prior art ridge waveguide structure shown inFIG. 2A to that of the present invention shown in FIG. 2B.

FIGS. 3A and 3B illustrate mode profiles of, respectively, a prior artstandard waveguide and a prior art ridge waveguide;

FIG. 3C illustrates a mode profile of a ride waveguide according to thepresent invention;

FIG. 4 illustrates a structure definition of a ridge waveguide coupledto a ring resonator, fabricated by a method according to the invention;

FIGS. 5A to 5E illustrate main manufacturing steps of the method usedfor fabricating the structure of FIG. 4;

FIG. 6 illustrates a structure definition of a two-layer ridge waveguideoptical coupler fabricated by a method according to the invention;

FIGS. 7A to 7E illustrate main manufacturing steps of the method usedfor fabricating the structure of FIG. 6;

FIG. 8 illustrates a structure definition of a ring resonator coupled toa low index difference waveguide, fabricated by a method according tothe invention;

FIGS. 9A to 9E illustrate main manufacturing steps of the method usedfor fabricating the structure of FIG. 8;

FIGS. 10A to 10C illustrate ring resonator based filters of,respectively, one, two and three rings, that can be manufactured by themethod according to the invention;

FIG. 11 illustrates the optical spectral response of the ring resonatorbased filters of FIGS. 10A-10C;

FIGS. 12A and 12B illustrate main constructional features and mainfunctional features, respectively, of a single channel Optical Add DropMultiplexer (OADM) according to the invention;

FIG. 13 illustrates a four port add drop multiplexer according to theinvention;

FIG. 14 illustrates an example of the integration of switches and adddrop filters for switch-able filters;

FIGS. 15A and 15B illustrate the construction and operational principlesof an interleave filter;

FIGS. 16A and 16B illustrate the construction and operational principlesof a single channel separation element (filter) according to theinvention utilizing ring resonators optically coupled to linearwaveguides;

FIG. 17 illustrates a spectral analysis filter according to theinvention;

FIG. 18 illustrates a tap coupler and spectral analysis system utilizingthe spectral analysis filter of FIG. 17;

FIG. 19 illustrates a spectrum analyzer according to the inventionutilizing several spectral analysis filters of FIG. 17; and

FIGS. 20A and 20B illustrate the construction and operational principlesof a sensor device utilizing an environmental sensitive filter accordingto the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 1A-1D illustrate the main principles of the prior art techniqueapplied for manufacturing the waveguide structure 10 shown in FIG. 1E.FIG. 1F illustrates the fabrication of the further structure 20 on topof the structure 10, so as to obtain a three-dimensional integratedoptical device. This technique will unavoidably require a planarizationprocedure to be applied to the uppermost layer of the first structure 10to allow the deposition of the second structure 20 thereon.

Referring to FIGS. 2A and 2B, a prior art ridge waveguide structure WS₁(FIG. 2A) is compared to a waveguide structure WS₂ of the presentinvention (FIG. 2B) According to the prior art techniques, a ridge 202was patterned from a core material—Layer 1, which is dielectric orsemiconductor material with a refractive index higher than that of theunderlying buffer layer (SiO₂). For example, the refractive indices ofLayer 1 and the underlying buffer layer may be 1.475 and 1.46,respectively. As a result, the height of the ridge 202 has to be asubstantial percent of the height of a waveguiding element (e.g., a 3-μmridge of the total 6-μm waveguiding element).

As shown in FIG. 2B, according to the technique of the presentinvention, a thin layer of a different material (dielectric orsemiconductor), is deposited on top of a layer 201 (Layer 1), and isutilized for fabricating a ridge 203 with a decreased height. Theresulting ridge part of the waveguide is much thinner in the verticaldimensions than the ridge part of state of the art ridge waveguide.

FIGS. 3A-3C illustrate the mode profiles (optical fields) of threedifferent light guiding structures. FIG. 3A shows the optical mode ofthe prior art waveguide structure 10 shown in FIG. 1E, FIG. 3B shows theoptical mode of the prior art waveguide structure WS₁ shown in FIG. 2A,and FIG. 3C shows the mode of the waveguide structure WS₂ of the presentinvention shown in FIG. 2B. The resulting mode profile in all cases isadaptable to fiber optic coupling, and hence can be used in integratedoptical circuits.

Referring to FIGS. 4 and 5A-5E, thee is illustrated one embodiment ofthe present invention. FIG. 4 shows an integrated optical device 30composed of a ridge waveguide 32 optically coupled to a ring resonator34. Ridge waveguides are commonly used in semiconductor-based opticalwaveguides, while being less commonplace in silica-based waveguides. Theintegrated optical device 30 can be used as a filter in signalprocessing applications. To this end, the coupling between the ringresonator 34 and a pair of buried channel waveguides is typicallyprovided, only the lower buried layer being shown in FIG. 4. The buriedchannels serve as input/output bus guides, while the ring functions asthe frequency selective element. At the resonance, power can besubstantially transferred from the input port to the drop port.

In the present example, the bottom layer waveguides are realized asburied ridge waveguides, with the ridge material being chosen as highindex material. FIGS. 5A-5E illustrate the main manufacturing steps of amethod according to the invention used for fabricating the structuredefinition 30. FIG. 5A shows an initial structure 35 obtained by thedeposition of a buffer layer 36 of a thickness about 10 μm onto asubstrate 38 (usually Silica over Silicon), and deposition of a layer 39of doped SiO₂, e.g., (SiO₂+Ge) onto the buffer layer 36. The layer 39has a refractive index higher than that of the buffer layer 36 (e.g.,1.475 of layer 39 compared to 1.46 of buffer layer 36). The depositioncan be of PECVD, LPCVD or another kind.

Then, a dielectric or a semiconductor layer 40 with a higher refractiveindex (e.g., in the range of 1.51-2.00), as compared to that of thebuffer layer 36, is deposited on the upper surface of the layer 39 (FIG.5B). Specific examples of such a dielectric material include, but arenot limited to, Silicone-OxyNitride, doped SiO₂, Ta₂O₅, poly silicon,amorphous silicon, Y₂O₃ (Yitterium Oxide).

The thickness of this guiding layer 40 in which a ridge 42 is formeddepends on the difference in the refractive indices of the dielectricand buffer layers. At a further manufacturing step (FIG. 5C), the ridge42 is created in the layer 40. According to the present invention, ahigh refractive index dielectric or semiconductor material is used tocreate the ridge. The height of the ridge is designed so that, after thedeposition of the next cladding layer, the combined perturbation istypically smaller than 200 nm.

To create the ridge 42, the ridge material 40 (dielectrical orsemiconductor) is covered by photoresist material (not shown), thewaveguide surroundings are exposed, and etching is carried out to definethe ridge(s) for the waveguide(s). It should, however, be noted thatthese steps can be replaced by a lift off procedure, wherein, first, thephotoresist is applied and the waveguide structure is exposed, and,then, the ridge material is deposited.

Thereafter, a further buffer (serving as a cladding) layer 44 isdeposited (FIG. 5D), which may be of the same composition as the firstbuffer layer 36. A height difference Δh of the height of a region oflayer 44 above the ridge 42 and a region of layer 44 outside the ridge42 (FIG. 4) is small, typically equal to or less than the height of theridge (e.g., less than 200 nm). This step thereby results in that thetop layer 44 is flattened to less than 200 nm, which is suitable for thedirect formation of an additional waveguiding structure, i.e.,dielectric ring resonators 34 in the present example (only one suchresonator being shown here). To this end, a high index dielectric orsemiconductor material is deposited (such as Si₃N₄, Ta₂O₅, Si), andmasking of the ring structure 34 is carried out by photolithography.Then, the ring structure undergoes anisotropic etching. Finally, afurther burying layer 46 is deposited, usually with the same compositionas the buffer layer 36 (i.e. SiO₂). By this, significant perturbationsof light propagation in the additional waveguiding structure areprevented.

Reference is now made to FIGS. 6 and 7A-7E illustrating athree-dimensional optical coupler 50 formed by a combination of anoverlay layer with a waveguide structure (FIG. 6), and the mainmanufacturing steps in a method of its fabrication (FIGS. 7A-7E). Tofacilitate understanding, the same reference numbers are used foridentifying those structure elements, which are common in the examplesof FIGS. 5A-5E and 7A-7E.

FIG. 7A-7D show manufacturing steps which are similar to those of FIGS.5A-5D. Namely, the dielectric or semiconductor layer 40 is deposited onthe stack-structure 35 formed of the substrate layer 38 (Si), bufferlayer 36 (SiO₂) and doped silicon layer 39 (SiO₂+Ge). The guiding layer40 has a higher refraction index (e.g., in the range of 1.51-2.00), ascompared to the index of refraction of the buffer layer 36, thethickness of the layer depending on the index of refraction difference.This guiding layer may be formed from Silicone - OxyNitride, doped SiO₂,Ta₂O₅, YO (Yitterium Oxide). Then, the ridge 42 is created in the highrefraction index layer 40 (FIG. 5C), and the buffer layer 44 isdeposited on the outer surface of the so-obtained structure (FIG. 5D).

At further manufacturing steps, the following procedures are carriedout: Further doped silicon layer 48 (SiO₂+Ge) and dielectric layer 49are sequentially deposited. The dielectric layer has either a slightlyhigher refraction index than the buffer layer (e.g., SiliconeOxyNitride, doped SiO₂, Ta₂O₅, YO), or a much higher refraction index(e.g., Si₃N₄ with the index of refraction being 2.00). Photolithographyand etching are sequentially applied for, respectively, masking anddefining a waveguide 52. Finally, a burying layer 54 (e.g., SiO₂) isdeposited.

An alternative embodiment of the invention relates primarily to couplingof waveguides and ring resonators. According to this technique, the ringstructures are first fabricated (which are relatively thin due to theirhigh refractive index), and then the regular waveguides are defined inthe overlay structure. By utilizing relatively broad waveguides with asmall refractive index difference, a higher tolerance to potentialperturbations in the waveguide structure can be achieved, as aconsequence of the residual perturbations of the underlying ringstructure.

The above approach is exemplified in FIGS. 8 and 9A-9E, wherein FIG. 8illustrates a complete structure 60 fabricated by sequentiallyperforming the steps shown in FIGS. 9A-9E.

Initially, a buffer layer 36 is deposited on a substrate 38 (usuallySilica over Silicon), using PECVD, HPCVD or other deposition methods.Deposited on the buffer layer 38 is a dielectric layer 40 with a muchhigher refraction index (e.g., Si₃N₄). The thickness of the guidinglayer 40 depends on the refractive index difference, but can be a fewhundred nanometers.

Photolithography is performed to mask the waveguides of the ringstructures 34, and anisotropic etching is applied to define thewaveguides for the ring structures (FIG. 9C).

Thereafter, a SiO₂buffer layer 62, and a dielectric 64 with a slightlyhigher refractive index (e.g., in the range of 1.48-1.51) than that ofthe SiO₂ layer are sequentially deposited. This dielectric material may,for example, by Silicone OxyNitride, or SiO₂ with a Ge or other doping.Then, the deposition of a photoresist material (not shown), andsubsequent exposure and development are carried out to define thewaveguides 68. By performing anisotropic etching of the guiding layer,the waveguides are defined. A further SiO₂ upper cladding layer 66 isdeposited on the so-obtained structure (FIG. 9E) to enable thefabrication of a further layer architecture of the entire structure 60(FIG. 8).

The following are some possible examples of integrated optical devicesthat can be manufactured by the above-described methods. It should benoted, although not specifically shown in the figures, that all devicesmanufactured by the invented technique can be tuned or switched bychanging the device temperature using the thermooptic effect. This canbe implemented by depositing microresistors (made of Cr, Ni—Cr,PolySilicone, etc.) on top of the multi-layer structures and applyingelectric current to the microresistors.

FIGS. 10A-10C illustrate ring resonator based filters of, respectively,one, two and three rings. As shown, one or more ring waveguides areaccommodated between two linear waveguides, each ring waveguide beingoptically coupled to the linear waveguides. Each ring resonator is anoptical filter, and, by combining them in parallel, high order filterscan be obtained. The examples of FIGS. 10B and 10C present a combinationof two spaced-apart linear waveguides and two and three spaced-apartresonator-cavity loops, respectively. The resonator-cavity loops areaccommodated between the two linear waveguides and are connected to eachother through sections of the linear waveguides, such at least twospaced-apart resonator-cavity loops and the waveguides sections creatinga closed loop compound resonator for storing optical energy of apredetermined frequency range. The physical characteristics of thecompound resonator are controllable (by an external field) to adjust theoptical storage characteristics of the compound resonator.

FIG. 11 illustrates the optical spectral response of the ring resonatorbased filters. Graphs G₁, G₂ and G₃ correspond, respectively, to one-,two- and three-ring filters.

Optical Add Drop Multiplexer

FIGS. 12A and 12b illustrate main constructional features and mainfunctional features, respectively, of a single channel Optical Add DropMultiplexer (OADM), generally designated 70. The OADM 70 is composed oftwo compound resonators 72 and 74, each constructed as described above,namely, including two ring-resonators accommodated between and coupledto two linear waveguides. Here, each ring resonator is an opticalfilter, and, by combining them in parallel, high order filters areobtained. The drop port (filter) is implemented using a double filterpass, while the add port is obtained with a single filter.

FIG. 13 illustrates a four port add drop multiplexer. Here, multiplechannel OADMs are obtained by cascading the structures of FIGS. 12A-12B.In a real system, four blocks D are followed by four blocks A.

FIG. 14 illustrates an example of the integration of switches and adddrop filters for switch-able filters. Here, optical switches are addedto insert and extract the ring based OADM from the optical path to allowhitless operation - not disturbing the through channels while tuning theOADM filters.

Interleaved Filters

Interleave filters are typically employed to achieve tight channelspacing in optical communication systems. As shown in FIG. 15A, such afilter accepts an incoming signal composed of optical channels with asmall frequency spacing and distributes the input channels amongstoutput waveguides in a circular function. Spectral functionality of thisinterleaved filter is shown in FIG. 15B. The output frequencies have awider frequency spacing, resulting in wider tolerances from the opticalelements. Interleaved filters are critical elements in achieving verytight frequency spaced optical channels.

Ring resonators are ideal candidates for implementing an interleavefilter in a planar lightwave circuit. A particular embodiment isobtained by using a ring resonator with a free spectral range (FSR),which is equal to the desired output frequency spacing. FSR is thefrequency at which the response of an optical filter repeats itself.

FIG. 16A illustrates a single channel separation element (filter)utilizing ring resonators optically coupled to linear waveguides. Asshown in FIG. 16B, by combining four single stage filters (SSF) of FIG.16A, the interleaved filter of FIG. 15A can be obtained.

Optical Spectrum Analyzer

The real time monitoring of optical networks poses the followingchallenges for spectral analysis systems: high resolution; shortspectrum acquisition time; low cost; low loss on the optical link; andsmall size. In standard spectrum analyzers, where the wavelengthseparation element is based on gratings, high resolution implies largersize and higher cost. An alternative would be to use tunable filters toscan across the optical spectrum of interest. However, existing tunablefilters are limited in their ability to provide the required resolution.

The technique of the present invention provides for manufacturing acompound high Q optical ring resonator structure operable as a filter,which is used in the analysis of optical spectra.

FIG. 17 illustrates a spectral analysis filter 80 associated with adetector 82. The filter 80 comprises two compound resonators 80A and 80Bconnected in parallel through a common linear waveguide W₂, and servesas a compound high Q optical ring resonator structure. The output linearwaveguide W₃ of the structure is connected to the detector 82. The Q ofthe filter is determined by the coupling factor describing the amount oflight that is coupled into the filter at every round trip. The Q factoris also determined by the optical losses in the cavity and the ringradius.

FIG. 18 illustrates a tap coupler and spectral analysis system,generally designated 90, utilizing the combination of the spectralanalysis filter 80 and detector 82. The filter 80 is connected to anoptical network (link) 84 via a coupler 86, which taps a small amount oflight, thereby minimizing the losses incurred in the optical link.

FIG. 19 illustrates a spectrum analyzer 100 utilizing several spectralanalysis filters—three such filters 110A, 110B and 110C in the presentexample, used in parallel through a common input linear waveguide W₁.Each filter has a different radius, as compared to the others, andtherefore is capable of carrying out a spectral analysis on a differentportion of the communications spectrum. This feature is associated withtwo problems that may occur when using ring resonators, namely, limitedtuning range and limited free spectral range, resulting in that adifferent approach has to be adopted to scan across a wide spectrum. Inaddition, this parallel approach provides for reducing the scan timerequired to approach all frequency channels of the communicationsspectrum.

Sensor

A high Q optical filter can be used as a very sensitive sensor forvarious applications, such as a biological, mechanical, or temperaturesensor. To this end, the filter characteristics depend on the externalelement to be measured. FIG. 20A illustrates a sensor device 200comprising an environmental sensitive filter 210 constructed as theabove-described compound resonator, which is connected to a laster 212and a detector 214 through its input and output waveguides. W₁ and W₂,respectively. Such a high Q optical filter structure is used as asensor, which is suitable for various applications, such as abiological, mechanical, or temperature sensor. This is due to the factthat the filter characteristics depend on the external element to bemeasured. FIG. 20B shows the results of tuning the laser 212 to the edgeof the filter 210. Generally speaking, the environmental element changesthe resonance frequency of the filter, which results in a change of theoptical power at the detector. With proper calibration, this device canbe used to measure or monitor various physical, mechanical or biologicalenvironmental changes.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the preferred embodiments ofthe invention as hereinbefore exemplified without departing from itsscope defined in and by the appended claims.

What is claimed is:
 1. An integrated optical device comprising at leastone structure having at least one waveguiding element, the devicecomprising: a basic structure containing a substrate material carrying abuffer material layer coated with a core material layer of a higherrefraction index as compared to that of the buffer layer; said at leastone waveguiding element formed in a guiding layer on top of said basicstructure, wherein said guiding layer is made of a material with arefractive index higher than the refractive index of said buffer layerand the core layer, and is chosen so as to minimize a height of said atleast one waveguiding element and to provide effective guiding of lightin the core layer; a cladding layer on top of a structure with said atleast one waveguiding element, wherein a height difference between aheight of the top cladding layer region above said at least onewaveguiding element and a height of the cladding layer region outsidethe waveguiding element is substantially small, the top cladding layerthereby having a desired flatness.
 2. The device according to claim 1,operable as an optical frequency dependent switch.
 3. The deviceaccording to claim 1, operable as an optical tunable filter.
 4. Thedevice according to claim 1, operable as an Optical Add Drop Multiplexer(OADM).
 5. The device according to claim 1, operable as a spectralanalyzer.
 6. The device according to claim 1, operable as a sensor. 7.The device according to claim 1, further comprising an additionalstructure formed on top of said cladding layer and including at leastone waveguiding element, the device being thereby a three-dimensionalintegrated optical device formed of the lower and upper structures, eachincluding at least one waveguiding element.